We had an unopened half gallon of apple cider in the fridge. I've used this local orchard's cider to make hard cider in the past as it is very tasty, fresh and UV pasteurized only, but this one was just to drink as is (or was)
Well, it was forgotten behind some other containers for about a week, and yesterday my wife pulled it out of the fridge... it was much more round than before
I opened the lid (actually it blew off after getting most of the safety seal peeled off) and put an airlock on it. It's been happily bubbling away since. I was somewhat surprised that it was fermenting so vigorously in the 36°F fridge. I suppose there's no [free or cheap] way of determining what yeast(s) are fermenting this?
Since putting the airlock on, it's been in a 59-60°F room, but now I'm wondering if I should have left it in the 36°F to see how it would have turned out.
Obviously I didn't get an OG reading since it was still sealed. I suppose I could guesstimate OG by picking up another bottle of fresh cider there and measuring it, although I'm sure batches vary quite a bit at this small local orchard.